Do you know how your touch screen works?

In this episode, you’ll learn about an important electrical effect called capacitance that can be used like batteries, to control timing, and even to detect touches.

Listen to the full episode for more explanation about how capacitance can be used to detect when and where a person touches a screen. Or you can also read the full transcript below.

Transcript

In this episode, you’ll learn about an important electrical effect called capacitance that can be used like batteries, to control timing, and even to detect touches.

Capacitance is the ability to store a charge. Not like how a battery produces charge through a chemical reaction but by storing positive and negative charge directly.

Let’s say you have a piece of metal. You can push extra electrons into the metal or draw electrons out creating either a negative or positive charge. But at some point it gets harder to move the charge.

Think of an empty bus that you try to fill with people. At first, it’s easy for people to board but soon the bus becomes crowded and people start resisting other people getting too close. It doesn’t matter what shape the bus is just like it doesn’t matter what shape the metal is.

You can fit more people on the bus though if they all cooperate and don’t mind being so close. But what can you do about electrons?

A positive or negative charge creates a static electric field which will attract opposite charge just like how opposite magnetic fields attract each other. We can use this to pack electrons tighter and to remove more electrons.

All we need to do is fill one piece of metal with positive charge and another with negative charge. Then to get more charge, put the two pieces of metal close together but not touching so that the static electric field can reach into the other piece of metal and help pull more of the opposite charge into the other side. Both sides will end up helping fill the other side with charge.

But there’s only so much that an electric field can help and it gets weaker the farther away you go. Because the shape doesn’t matter, we can make the pieces of metal flat so more area can be placed next to the other side. Because the sides are flat, they’re called plates.

That’s all that really goes into making a capacitor. I mentioned that the sides shouldn’t touch because if they do, then the stored charge will equalize between the two sides and maybe ruin the capacitor. There’s a separator in capacitors called the dielectric. It’s really just a material that can stop direct current from flowing while letting electric fields pass easily. This material can even be air.

Capacitance is measured in farads and a one farad capacitor will be able to store one coulomb of charge and produce one volt between the two plates. Most capacitors are much smaller than a farad and are measured in microfarads which are millionths of a farad or picofarads which are billionths of a farad. But it is possible, especially with modern manufacturing to make large capacitors of a farad or more.

If you ever need to open an electronic device you need to be careful of large capacitors because even if the device is unplugged, the capacitors can still hold a tremendous amount of charge. This charge is ready to move immediately and doesn’t need to wait for a chemical process to produce more charge. I’ve been shocked with capacitors before and my whole arm ached for a week.

Capacitors can also be filled with charge rapidly. Again, there’s no need to wait for any chemical reaction. At the speed that modern computers operate, it’s possible for some integrated circuits to need more power than can travel from the power supply fast enough. Capacitors can act like little storage units spread throughout the printed circuit boards. They can even be placed inside integrated circuits to help make sure that power is available whenever it’s needed.

As a capacitor fills up with charge, the voltage will increase. A fully discharged capacitor will have zero volts and that will increase over time as the capacitor fills up. Imagine that bus again. When empty, people can board fast. When the bus is half full, the rate will slow down. When the bus is almost full, it can take a lot longer for even a single person to board the bus.

Now, if you connect a capacitor directly to a power supply it’ll charge almost immediately. Much too quickly to use for timing. But by putting a little extra resistance in line with the capacitor, we get something called the RC time constant. If you multiple the resistance by the capacitance and interpret that as a unit of time, then the capacitor will almost fully charge to whatever voltage level is being used in 5 of these units of time. It takes just one of these units of time for the capacitor to charge to about 60%. The actual voltage doesn’t matter as long as it doesn’t exceed what the capacitor can withstand. Too much voltage will cause the dielectric to breakdown and allow current to flow directly between the plates.

So if you know the resistance and know the capacitance, then you’ll know how long it takes to charge a capacitor. As the capacitor charges, its voltage will rise. It’s possible then to measure the voltage and wait for it to reach a certain level. This time will be predictable and can be repeated.

Now let’s go back to what controls how much capacitance a particular capacitor has. If the size of the plates are increased, then they can hold more charge and the capacitance increases. If the plates are moved closer together, then the capacitance also increases because the static electric field will be stronger the closer the plates get to each other. And if the dielectric material changes so that it lets the electric field pass through better, then the capacitance increases.

The dielectric material is probably not going to change but it can sometimes maybe by pushing and pulling some material in or out of the capacitor.

Surely the size of the plates will remain fixed, right? I mean how can you suddenly add more plate material? What’s important is actually the area of overlap between the plates. If the plates can rotate so they can slide past each other then the capacitance can be changed. This is actually how old fashioned radios were tuned to different stations.

I mentioned at the beginning of this episode about touch screens and how they can sense when and where they’re touched. How does this work?

They can sometimes work differently. One way is to use our bodies ability to store charge to affect the electric field of capacitors. When we bring our finger close to a capacitor plate, the electric field will change and the capacitance will change.

By rapidly charging and discharging a capacitor and watching for changes in the time it takes, then we can detect when a finger is nearby. While this can be used to turn on or off a lamp, how can it be used to determine exactly where on a phone’s screen a person is pointing? One way to do this is to divide the screen into rows and columns of separate plates. The plates are actually behind the glass screen. This creates an array of capacitors that can be cycled through to detect position.